Within the communications industry gridded vacuum tube amplifiers have been proposed for use in a number of fields of application. Electron valve devices, including microwave vacuum tube devices in the form of power amplifiers, for example, can be utilized as components of microwave systems including telecommunications, radar, electronic warfare, and navigational systems, among others. In some instances, semiconductor microwave amplifiers do not provide the power capabilities utilized by some microwave systems. However, in some instances, vacuum tube amplifiers can provide microwave power which is higher by orders of magnitude. The higher power levels can be achieved, for example, because electrons can travel faster in vacuum with fewer collisions than in semiconductor material. These higher speeds can, in some instances, permit larger structures with the same transit time which, in turn, can produce greater power output.
Other electron valve devices can include, for example, triodes, tetrodes, pentodes, klystrodes, klystrons, traveling wave tubes, cross-field amplifiers, and gyrotrons. In addition, the devices can have various sizes such that they can be used in various applications, including, for example, small devices for use in audio signal amplification. Such devices can contain a cathode structure including a source of electrons, an interaction structure (e.g., grid or gate), and an output structure (e.g., anode).
In such devices, the source of electrons can be a thermionic emission cathode. The emission cathode can be formed, for example, of tungsten that can be coated with barium or barium oxide, or mixed with thorium oxide. Thermionic emission cathodes can be heated to temperatures around one thousand (1000) degrees Celsius (° C.) to produce sufficient thermionic electron emission current (e.g., on the order of amperes per square centimeter). The heating of thermionic cathodes to such temperatures can reduce or limit the lifetime of the cathodes, introduce warm-up delays, require auxiliary equipment for cooling, and/or interfere with modulation of emission, for example, in gridded tubes. In addition, as vacuum tube size decreases, high temperature thermionic emission cathodes can present increase heat and reliability problems.